One type of an operational amplifier which produces balanced output is described on page 314 of "1983 IEEE International Solid State Circuits Conference, Digest of Technical Papers". In a circuit for the operational amplifier, a differential amplifying transistor pair including two N channel MOS transistors (N channel MOS transistor is called simply as "MN" hereinafter) is connected to a constant current source including a MN, to a gate electrode of which a constant voltage is applied, and two parallel MNs, to gate electrodes of which an analog ground voltage is applied. In the differential amplifying transistor pair, gate electrodes of the two MNs are connected to balanced input terminals, while drain electrodes for outputs thereof are connected to source electrodes of a folded cascade transistor pair including two P channel MOS transistors (P channel MOS transistor is called simply as "MP" hereinafter). Drain electrodes of the folded cascode transistor pair are connected to a constant current source including four MNs.
In the circuit of the operational amplifier, an amplifying circuit is provided by one stage, so that a gain is obtained in accordance with a resistance value of a load. Therefore, there are disadvantages that a load of a large capacitance is difficult to be driven because an ability for driving a capacitive load is limited due to a mutual conductance of the input differential amplifying transistor pair and a capacitance value of the load, that a current consumption is large because a current flowing through the constant current source which is a bias current for the operational amplifier must be large to increase a lowering rate of an output voltage which is defined as a slewing-rate for the reason why the slewing-rate is limited due to a current value for the constant current source, and that a range of an output voltage is narrow because the number of MOS transistors is large to be connected in series between two power supply lines. These disadvantages are detrimental to an operational amplifier which is used for a large scaled integration circuit.
The other type of an operational amplifier which includes two-stage amplifying circuits to enhance an output driving ability is described on pages 919 to 925 of "IEEE Journal of Solid-State Circuits, Vol. SC-19, No. 6, Dec. 1984". In a circuit of the operational amplifier, a differential transistor pair including two MPs is connected to a constant current source of a MP, such that source electrodes of the two MPs are commonly connected to the constant current source, and a constant voltage is applied to a gate electrode of the MP for the constant current source. Input terminals are connected to the differential transistor pair, drain electrodes of which are connected to, source electrodes of two MNs for a folded cascode stage constructed together with two MPs of a current mirror circuit. In the folded, cascode stage, gate electrodes of the two MNs are connected to a reference voltage input terminal, and source electrodes of the two MPs are connected to a first power supply line. The current mirror circuit is connected to a constant current source including two MNs, source electrodes of which are connected a second power supply line. Output of the folded cascode stage is amplified in an inverting amplifier including a MP for a driving transistor and a MN for a constant current load, and then supplied from an output terminal of the operational amplifier to a following stage.
In the operational amplifier, a range of an input voltage can be large, and an output voltage can be lowered down to a voltage approximately equal to a negative voltage of the second power supply line.
According to the latter operational amplifier, however, there are disadvantages that a current consumption is large because a current flowing through the MN for the constant current load must be large to increase a slewing-rate of lowering an output voltage in a case where a capacitive load is connected thereto and an output signal of a large amplitude is supplied therefrom, and that an output voltage can not be increased up to the upper level of a power supply voltage because the output voltage is limited due to the aforementioned current and a resistance of the MP which is turned on for the driving transistor. These disadvantages do not allow the latter operational amplifier to be used for an operational amplifier realized on a large scaled integration circuit.